ERR_CERT_COMMON_NAME_INVALID on those setup links usually means the link is pointing at a host that hasn't finished cert provisioning yet, or the email sent you to the wrong URL for your enterprise.

With Enterprise Managed Users, your account lives on a dedicated subdomain like <yourenterprise>.ghe.com, not on github.com. Copy the full URL from the email (paste it into a plain text editor) and confirm the hostname matches your enterprise slug, something like https://<slug>.ghe.com/.... If the slug is off or the host looks internal, that's your issue.

If the URL looks right but you still get the cert warning, it's almost always a tenant that hasn't fully provisioned yet. I've seen this take anywhere from a few minutes to a couple of hours after the enterprise is created. Wait 30 to 60 minutes, open the link in an incognito window, and try again.

If it's still broken after that window, this isn't something the community can unblock. Open a ticket with GitHub Enterprise support via https://support.github.com and include the exact URL from the email plus a screenshot of the cert details (click the padlock, view certificate). They can see which host the invite was generated against.

One thing, don't hit "Proceed anyway" on that warning. If the cert is actually invalid, bypassing it will either fail the auth flow or land you somewhere you don't want to be.